Process for the production of lithographic printing plates utilizing peel development

ABSTRACT

A photosensitive material useful for the production of lithographic printing plates, which contains (a) a support suitable for lithographic printing plates, (b) a hydrophilic layer which contains a crosslinkable organic polymer or is a layer formed by crosslinking such a polymer, (c) a photopolymerizable layer and (d) a transparent cover film, the adhesion of the photopolymerizable layer (c) to the hydrophilic layer (b) and/or to the cover film (d) being changed by exposure. The material permits the production of lithographic printing plates having high printing performance without the use of liquid processing agents.

This application is a division of application Ser. No. 08/318,542, filedOct. 5, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensitive material which issuitable, for example, for the production of lithographic printingplates and contains

(a) a support suitable for lithographic printing plates,

(b) a hydrophilic layer comprising an organic polymer,

(c) a photopolymerizable layer, and (d) a transparent cover film.

The invention further relates to a process for the production oflithographic printing plates in which a material of the above mentionedgeneric type is exposed imagewise and is developed by mechanicallypeeling apart the transparent cover film and the support. This processis known as the "peel-apart" technique.

Materials and processes of this type have the substantial advantage thatthey require, for their processing, no developer solutions which have tobe disposed of in an environmentally acceptable manner. During theexposure, the adhesion of the photopolymerizable layer to the layersadjacent to it changes, so that, when the cover film and support arepeeled apart, the exposed parts of the layer adhere to one side and theunexposed parts to the other side.

2. Description of Related Art

DE-C 15 72 122 describes a process for the production of images, inwhich a material comprising a transparent film, a photopolymerizablelayer and a second film is exposed imagewise and is developed by peelingapart the film to give a negative image and a positive image.

EP-A 514,186 and U.S. Pat. No. 4,895,787 describe similarphotopolymerizable materials which are processed by peel-apartdevelopment and are used for the production of color proofing films.

The above publications describe materials in which a colorless orcolored photopolymerizable layer is arranged between two films adaptedto the particular intended use. Adhesion-promoting, release or colorlayers may be provided, if required, between the photosensitive layerand film supports or cover films. In each case, the exposure results ina change in the adhesion to the film support or cover film or to thelayer in between, as well as a change in the cohesion within thephotosensitive layer, so that imagewise separation within thephotosensitive layer .occurs by utilizing the adhesion and cohesiondifferences when peeling apart the films. Clean separation of theexposed and unexposed parts of the layer from the less adhesive film ispermitted by virtue of the fact that the films have relatively smoothsurfaces.

The production of lithographic printing plates by this process is moreproblematic since the nonimage parts must be detached from the surfaceof the printing plate support, which is usually a grained and, ifrequired, anodized aluminum sheet. Here, even very small residues of theoleophilic photosensitive layer which remain behind in the rough poroussupport surface would impair the usefulness, since residue leads toscumming in the nonimage parts. Replacement of the support by a materialhaving a smooth surface is not directly possible since the structuredhydrophilic surface of the support is required for conveying waterduring lithographic printing.

DE-A 27 25 762 describes photosensitive printing plates which areprocessed by peel-apart development. Between the printing plate supportand the photosensitive photopolymerizable layer, the materials have ahydrophilic intermediate layer which comprises, for example, awater-soluble silicate. This permits an improvement in layer separationduring development, i.e., good differentiation between oleophilic andhydrophilic parts of the surface of the printing plate. However, thehighly hydrophilic character of the intermediate layer makes itsusceptible to attack by aqueous solutions, so that, in the course ofthe printing process, the image elements of the oleophilic image layermay be underwashed and attacked or may even be completely detached fromthe support. Satisfactory print runs therefore cannot be achieved.

DE-A 29 23 980 describes a similar printing plate which has aphotosensitive intermediate layer based on diazo compounds and aphotopolymerizable layer thereon. This plate permits better anchoringand resistance of the image parts, but there is once again the danger ofincomplete removal of the diazo layer, particularly after relativelylong storage of the underexposed printing plates, so that here too theproblem of scumming occurs.

EP-A 530,674 describes a similar material which contains, between theprinting plate support and the photopolymerizable image layer, anintermediate layer comprising a hydrophilic polymer, for example,polyvinyl alcohol, and a polymerizable compound. During exposure, thepolymerizable compound takes part in the polymerization of thephotopolymerizable layer and ensures greater adhesion between theselayers. Here too, the polymerization in the intermediate layer evidentlydoes not offer sufficient resistance to washout during the printingprocess, so that satisfactory print runs are not achieved.

WO-A 93/5446 describes a material which is similar to that describedabove but which contains a hydrophilic photopolymerizable intermediatelayer having a relatively high proportion of monomers and ofphotoinitiator. Again, the intermediate layer is subject to the dangerof unintentional partial polymerization when it is handled duringprocessing after imagewise exposure in the daylight. In the absence ofspecial precautions, polymerization and hence the imparting ofoleophilic properties to the nonimage parts can readily occur, leadingto scumming. This layer is therefore preferably washed out with water orfountain solution prior to printing.

SUMMARY OF THE INVENTION

It is therefore an object of the invention provide a negative- orpositive-working photosensitive material useful, for example, for theproduction of lithographic printing plates, which can be processedwithout the use of liquid developers to give a lithographic printingplate having high printing performance and little tendency to scumming.

It is also an object of the invention to provide a process for theproduction of such photosensitive material and processes of producinglithographic printing plates using such a photosensitive material.

In accordance with these and other objects, there has been provided aphotosensitive material useful for the production of lithographicprinting plates which comprises (a) a support suitable for lithographicprinting plates, (b) a hydrophilic layer which comprises a crosslinkableorganic polymer or is a layer formed by crosslinking an organic polymer(c) a photopolymerizable layer, and (d) a transparent cover film,wherein the adhesion of the photopolymerizable layer (c) to thehydrophilic layer (b) and/or to the cover film (d) is changed byexposure.

In accordance with these and other objects there also has been provideda process for the production of a lithographic printing plate,comprising exposing a material as described above imagewise through thecover film (d) and developing by peeling off the cover film (d) from thesupport (a), wherein the polymer of the hydrophilic layer (b) iscrosslinked before or after exposure.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description which follows.

DETAILED DESCRIPTION

In the material according to the invention, the hydrophilic layer (b)contains a crosslinkable organic polymer or is a layer formed bycrosslinking such a polymer. Any organic polymer or polymers which arecrosslinkable or crosslinked can be used in the hydrophilic layer.

The term "crosslinkable" as used in the present Specification means thatthe polymer is either self-crosslinkable at elevated temperature or iscrosslinkable by heating in the presence of crosslinking agents. Suchcrosslinking may occur during the normal treatment conditions of theprocess. This means that the layer (b) must contain a crosslinkingagent, if the polymer is not self-crosslinkable.

The crosslinkable hydrophilic polymer used for the production of thehydrophilic layer (b) is usually water-soluble. Suitable polymers aredescribed by C. L. McCormick, J. Bock and D. N. Schulz in Encyclopediaof Polymer Science and Engineering 17, 730 (1985) the contents of whichare hereby incorporated by reference. The crosslinking of the polymersmay be achieved in any desired manner. Depending on the functionalgroups contained in the polymers, crosslinking may be achieved withthese polymers alone, for example, by heating. It is frequentlyadvantageous, however, to use low molecular weight or high molecularweight organic or inorganic crosslinking agents. Furthermore,crosslinking catalysts may be added to accelerate the reaction.

The proportion of the crosslinkable polymer in the layer (b) isgenerally in the range from 20 to 100, preferably from 50 to 100 andparticularly preferred from 70 to 99 per cent by weight.

Examples of suitable water-soluble crosslinkable polymers includenucleotides, polypeptides, polysaccharides, polyacrylamides,polyethylene oxides, polyvinyl alcohols, their copolymers or graftcopolymers, and the water-soluble derivatives of such polymers, whichderivatives are obtained by polymeranalogous reaction. Ionic polymers,such as polyamines, polyimines, polyvinylpyridines,polyvinylpyrrolidones, poly(meth) acrylic acids, polysulfonic acids andpolyphosphonic acids, may also be used. Furthermore, the amphotericacrylate copolymers described in DE-A 40 23 269 and 40 23 268, thecontents of both of which are hereby incorporated by reference mayadvantageously be used. Preferred polymers are those having vinylalcohol, vinylpyrrolidone and (meth)acrylic acid units.

Suitable crosslinking agents include any desired and useful to crosslinkthe polymer. Examples include water-soluble polyfunctional compoundswhich react, with the polymer generally only at elevated temperature,for example, polyhydric alcohols, such as glycols or oligoglycols,glycerol, trimethylolethane or trimethylolpropane, or pentaerythritol;polyfunctional aldehydes, such as glyoxal or glutaraldehyde; polybasicacids, such as oxalic acid or citric acid; oxo acid derivatives ofboron, of aluminum, of silicon, of phosphorus or of chromium; orpolyfunctional amines, such as ethylenediamine, oligoethyleneoligoaminesor polyethyleneimines.

Suitable crosslinking catalysts include any useful for catalyzing thecrosslinking, and, are mainly acids or bases, e.g., p-toluenesulfonicacid, sulfuric acid, sodium hydroxide solution, tertiary amines orquaternary ammonium bases. The catalysts are present in an amounteffective to catalyze the crosslinking.

The hydrophilic layer may furthermore contain wetting agents, levelingagents, fillers, dyes and UV absorbers in effective amounts to achievethe intended purpose.

The layer weight of the hydrophilic crosslinked or crosslinkable layeris selected to give the desired results and is generally in the rangefrom 0.001 to 1, preferably between 0.01 and 0.5, and in particular from0.05 to 0.3, g/m². The amount of crosslinking agent is chosen to givethe desired crosslinking and is generally in the range from 0 to 80,advantageously between 0 and 50, in particular between 1 and 30, % byweight, based on the amount of the crosslinkable polymer.

Many crosslinkable polymers or mixtures thereof with crosslinking agentsundergo a crosslinking reaction even under the drying conditions of theprocess of lithographic printing plate production, i.e., at temperaturesin the range of 60°-120° C. and in the course of about 1-10 minutes.Other polymers, for example, poly(meth)acrylic acids, remain essentiallyuncrosslinked under the drying conditions. They are then crosslinked inthe course of a thermal aftertreatment of the printing plate, forexample, by baking at about 180°-240° C. in the course of 1 to 25,preferably 5 to 20, minutes, and thus become insoluble.

Crosslinking of the layer (b) is, as a rule, desirable and evennecessary in order to achieve a long print run in lithographic printing,i.e., good resistance to fountain solution. The crosslinking may beachieved at any point in the process, and even the initially usedpolymer may be crosslinked. The solubility of the uncrosslinkedhydrophilic layer in water or strongly polar solvents generally presentsno problems during subsequent application of the photopolymerizablelayer since the latter is applied from organic solvents which do notdissolve the hydrophilic layer. The photopolymerizable layer (c) may beany desired photopolymerizable layer, and generally contains, in amanner known per se, (1) a polymeric binder, (2) a compound capable offree radical polymerization, and (3) a compound or a combination ofcompounds which is capable of initiating the polymerization of thecompound (2) under the action of actinic radiation.

The binder (1) should be compatible with the other components of thephotopolymerizable layer (c), i.e., no phase separation is permittedwithin the layer over the entire temperature range of production anduse. Furthermore, depending on the version (positive or negative type),either the unexposed or the exposed parts of the layer should exhibitgreater adhesion to the cover film (d) than to the hydrophilic layer (b)on the support (a). Conversely, the exposed and unexposed parts shouldadhere more strongly to the hydrophilic layer (b) on the support (a)than to the cover film (d). A high affinity to printing ink and chemicaland mechanical stability. within the printing process are likewisedetermined by, inter alia, the binder.

Any polymeric binder meeting the characteristics can be used. Examplesof suitable polymers include chlorinated polyolefins, for example,chlorinated polyethylenes and chlorinated polypropylenes; poly(meth)acrylates, polyacrylonitrile, polystyrene, polyvinyl chloride,polyvinylidene chloride, polybutadiene, polyisoprene, polychloroprene,chlorinated natural rubbers, polyvinyl acetates, polyvinyl acetals,polyesters, polyamides and polyurethanes, as well as the copolymers ofthe monomers on which these polymers are based. Chlorinated naturalrubbers, poly(meth) acrylates and their copolymers and polyamides arepreferably used. Members of the class of the polyvinyl acetals, inparticular the polyvinyl butyrals, are also preferably used. The amountof binder in the photopolymerizable layer may be varied to achievedesired results and generally is between 10 and 90%, preferably between20 and 70%, based in each case on the weight of the nonvolatilecomponents in the photopolymerizable layer.

The compound (2) capable of free radical polymerization may be any suchcompound and is usually an ethylenically unsaturated substance or acorresponding mixture of substances in the form of esters or amides ofacrylic, methacrylic, fumaric or maleic acid. Members having more thanone polymerizable double bond are preferred, for example, the esters ofthe stated acids with alkanediols, (poly/oligo)ethylene glycols,(poly/oligo)propylene glycols, (poly/oligo)butylene glycols and otherbifunctional low molecular weight or high molecular weight organicdiols. The esters of polyhydric alcohols, such as glycerol,trimethylolethane and trimethylolpropane and pentaerythritol,isocyanuric acid, its ethoxylated or propoxylated derivatives and dimersor oligomers of these compounds are particularly suitable. As high adegree of esterification as possible is advantageous. Amides, forexample, the compounds which are formally formed in the reaction ofethylenediamine or its oligomers with the acids described, may likewisebe used. The amount of the monomers may be varied to achieve desiredresults, and is generally about 5 to 80, preferably about 10 to 60, % byweight of the nonvolatile components.

Depending on the desired sensitization range, various materials areuseful as photoinitiators (3). For example, if the photo-sensitivematerial is to be sensitive to the near UV range (350-420 nm) usuallyused in offset printing, the photoinitiator systems used differ fromthose employed when it is intended to carry out exposure in the visiblespectral range, for example, using laser radiation. Any desiredphotoinitiator or combination of photoinitiators which give the desiredresults may be used.

The photoinitiators to be exposed in the near UV range should absorblight in the range between about 250 and 500 nm with formation of freeradicals. Examples include acyloins and their derivatives, such asbenzoin, benzoin alkyl ethers, vicinal diketones and their derivatives,e.g., benzil, benzil acetals, such as benzil dimethyl ketal,fluorenones, thioxanthones, polynuclear quinones, acridines andquinoxalines, and trichloro- methyl-s-triazines,2-halomethyl-4-vinyl-1,3,4-oxadiazole derivatives, halooxazolessubstituted by trichloromethyl groups, carbonylmethylene heterocyclescontaining trihalomethyl groups and acylphosphine oxide compounds.

The photoinitiators can also be used in combination with one another orwith coinitiators or activators, for example, with Michler's ketone andits derivatives or 2-alkylanthraquinones.

In the case of the photoinitiators used, in particular for laserirradiation in the visible range, mixtures of metallocenes,photoreducible dyes, photolytically cleavable compounds havingtrihalomethyl groups and, if necessary, further initiator components anddye components are generally suitable, as described, for example, inEP-A 364 735, the contents of which are hereby incorporated byreference, are useful. The metallocene component comprises variouslysubstituted cyclopentadienyl complexes of titanium or of zirconium.Xanthene, benzoxanthene, benzothioxanthene, thiazine, pyronine,porphyrin or acridine dyes can be used as photoreducible dyes. In thecase of the trihalomethyl coinitiators, the known triazine derivativescontaining bromine or chlorine as halogen have proven particularlyuseful. Further initiator components are compounds which increase thesensitivity especially in the near UV range, for example, acridine,phenazine or quinoxaline derivatives, or which increase the sensitivityin the visible range, for example, dibenzylacetones or cumarins.

The amount of photoinitiator or of the initiator combination may bevaried to achieve the desired results, and is generally about 0.1 to 15%by weight, preferably about 0.5 to 10% by weight, of the nonvolatilelayer components.

The photopolymerizable layer may further contain stabilizers forsuppressing thermal polymerization, pigments, dyes, plasticizers orother auxiliaries for improving mechanical or reprographicquality ineffective amounts. In principle, it should be ensured that the addedsubstances do not absorb an excessive proportion of the actinic lightrequired for the crosslinking and hence reduce the photosensitivity inpractice. Where a thermal aftertreatment step is to be carried out inorder to increase the stability of the layer, additional components maybe present for the thermal curing thereof.

The layer thickness of the photopolymerizable layer (c), expressed bythe layer weight may be varied to achieve desired results and isgenerally 0.1 to 10, preferably 0.5 to 5.0, g/m².

Suitable flexible transparent cover films (d) for the material accordingto the invention include all known in the art and are in particularplastics films which are dimensionally stable on heating to about60°-130° C. For example, films of cellulose acetate, polystyrene,polyamides, polycarbonates, polyesters and polyimides are suitable forthis purpose. The film should preferably have little permeability toatmospheric oxygen. Films of polyesters, polycarbonates, polyimides andsimilar polymers are therefore preferably used; polyester films aregenerally preferred. The cover film may have any thickness which givesthe desired parameter, and generally is in the range from about 10 to100, preferably 12 to 50, μm. In order to improve the dimensionalstability, the films may be biaxially stretched and, if necessary,heat-set.

In order to improve the adhesion of the photopolymerizable layer (c) tothe cover film (d), the surface of the cover film can be subjected to anadhesion-increasing treatment, for example, by corona discharge, byetching with chemicals, such as trichloroacetic acid, and/or by coatingwith an adhesion-promoting lower layer. Such coatings are generally0.001 to 0.1 μm thick. They may comprise copolymers of (meth)acrylates,as described, for example, in U.S. Pat. No. 4,098,952 the contents ofwhich are hereby incorporated by reference, and may preferably becrosslinked. Suitable cover films are described in U.S. Pat. No.5,049,476, the contents of which are hereby incorporated by reference.The surface of the film also may be subjected to a treatment whichprevents blocking, for example, by coating with a suitable lower layerwhich contains finely divided organic or, in particular, inorganicparticles, the size, amount and refractive index of which are such thatthe transparency of the film is not adversely affected. The surface maybe smooth or matte. Useful films are described, for example, in EP-A130,222, the contents of which are hereby incorporated by reference.

The support (a) may be any desired support that is useful, for example,a support such as those used in the production of lithographic printingplates, and is selected depending on the use of the material. Forexample, the support may be a metallic sheet usually used as supportmaterial for lithographic printing plates and having a hydrophilicsurface. A preferred metal is aluminum, which preferably issuperficially grained and anodically oxidized. The surface also may havebeen treated in a known manner with alkali metal silicates, phosphates,hexafluorozirconates, polyvinylphosphonic acid or other conventionalpretreatment agents. The thickness of the support may be varied toachieve desired results, and is generally between 0.05 and 1, preferablybetween 0.1 and 0.5, mm.

The photosensitive material may be prepared in any desired manner. Forexample, it may be prepared by a procedure in which the components ofthe hydrophilic layer (b) are dissolved in a suitable solvent, generallywater or a mixture of water and water-miscible organic solvents, such aslower alkanols, acetone or the like, and are applied to the surface ofthe support (a) so that, after drying, the desired layer thickness isobtained. The photopolymerizable layer (c) is then applied to the driedintermediate layer, generally also by coating from a solution. Thesolvent is chosen so that the hydrophilic intermediate layer (b) is notdissolved. The cover film (d) is then applied to the photopolymerizablelayer (c) by lamination. Lamination may be effected at room temperatureor at elevated temperature.

The photopolymerizable layer may also be applied to the cover film bycoating from a solution, and may be dried there and then applied bylamination to the support (a) coated with the hydrophilic layer (b).This lamination process may also be carried out at a later time afterstorage of the elements comprising (a)+(b) and (c)+(d). In this case,however, it is expedient to cover the bare side of thephotopolymerizable layer (c) with a removable protective film, forexample, of polyethylene, which is then peeled off only immediatelybefore the lamination.

The photosensitive material obtained is exposed imagewise in any knownmanner through the transparent cover film (d). Exposure may be effectedby means of a correspondingly controlled radiation source, for example,a laser, or in contact under a transparent original. As a result of theimagewise crosslinking of the photopolymerizable layer, its adhesion tothe cover film (d) and/or to the hydrophilic intermediate layer (b) ischanged so that, when the cover film is peeled off, either the exposedor the unexposed parts of the photopolymerizable layer are removed withthe cover film. A positive image remains on the support (a) in the firstcase, and a negative one in the second case. The adhesion behavior whichgives either a positive or a negative image can be controlled by meansknown in the art.

Since generally, the adhesion of the photopolymerizable layer to therough surface of the support (a), which is not leveled by the relativelythin hydrophilic layer (b), is greater than that to the transparentcover film (d), this cover film should as a rule undergo anadhesion-enhancing surface treatment, as described above.

In most cases, the photopolymerization results in an increase in theadhesion of the layer (c) to the rough surface of the support (a), whichis covered by the intermediate layer (b). A change in this behavior canbe achieved by changing the composition, in particular the type ofbinder and/or polymerizable compound, of the layer (c); or thecomposition of the hydrophilic layer (b). In many cases hydrophiliclayers whose hydrophilic properties are due to the presence of ON groupsare used to change the adhesion at the interfaces of (b) to (c) byexposure so that a negative image is obtained. In other cases in whichthe layer (b) contains, for example, carboxyl groups or other acidgroups, positives are frequently obtained.

In general, the nature of the photopolymerizable layer has a relativelymajor effect on the copies obtained. Predominantly negative copies areobtained with layers which contain conventional free radical initiatorsand binders based on (meth)acrylate polymers, whereas, as a rule,positive copies are obtained with layers which contain polyvinyl acetalsas binders and initiators which form acids on exposure. Techniques forinfluencing the adhesion behavior are known and are described, interalia, in the article "Photosensitive Materials by Peel-off Development"by T. Ikeda, T. Yamaoka and T. Tsunoda in Graphic Arts Japan, Vol. 21(1979-1980), pages 26-31, the contents of which are hereby incorporatedby reference.

After development of the image, the image remaining on the support canbe thermally postcured or baked in a manner known per se, in order toincrease the length of the print run. Particularly in the case ofpositive printing plates in which the unpolymerized parts of the layerremain behind on the support, it is advisable to cure the screen byheating and/or postexposure.

The material and process according to the invention make it possible toobtain lithographic printing plates by a processing method for which noliquid developers or other processing solutions are required. Comparedwith known processes of this type, the process according to theinvention has the advantage that it gives printing plates which permit asubstantially longer print run.

The following examples illustrate preferred embodiments of the inventionwithout limiting the invention. Unless stated otherwise, amounts andpercentages are to be understood as weight units. The amounts aregenerally stated in parts by weight (pbw).

EXAMPLE 1

a) A solution of the composition stated below was applied to a 50 μmthick biaxially stretched and heat-set polyethylene terephthalate filmwhose surfaces on both sides had been pretreated to improve the adhesion(Melinex® 505):

2.36 pbw of polyethyl methacrylate (Tg 65° C., acid number 8),

0.05 pbw of polymethyl methacrylate (Tg 105° C., acid number<1),

2.15 pbw of trimethylolpropane triacrylate,

0.08 pbw of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one,

0.25 pbw of Renol Blue B2G (C.I. 74 160),

0.09 pbw of 4-diethylamino-4'-methoxydibenzalacetone and

0.03 pbw of 2,6-di-tert-butyl-4-methylphenol in

49.8 pbw of butatone,

2.5 pbw of tetrahydrofuran,

35.6 pbw of propylene glycol monomethyl ether and

7.13 pbw of γ-butyrolactone

After drying, the layer weight was 1.91 g/m².

b) A 0.3 mm thick electrolytically grained and anodically oxidizedaluminum sheet having an oxide layer weight of 3.6 g/m² was aftertreatedwith a 0.1% strength aqueous polyvinylphosphonic acid solution. Anaqueous solution of 0.9% of a partly hydrolyzed polyethyleneglycol/vinyl acetate graft polymer having an ester number of 150, 0.1%of glyoxal and 0.01% of p-toluenesulfonic acid was applied to thissupport by spin coating, so that a layer weight of 0.2 g/m² resultedafter drying.

c) The photopolymerizable material described under (a) was laminated,under pressure at 90° C., with the support described under (b). Thephotosensitive printing plate obtained was exposed to a 5 kW metalhalide lamp at a distance of 110 cm for 12 seconds in contact under atest original. The polyester film was then peeled off manually, theunexposed parts of the layer being removed cleanly with the film and theexposed parts of the layer remaining behind together with the curedpolyvinyl alcohol layer on the aluminum sheet as a negative copy havinggood resolution. The negative image was postexposed without an originalfor 100 seconds and, in order to test the resistance to washing out, wastreated in an ultrasonic bath with water (1), with 1% strength aqueouspolyethylene glycol-(300)-nonyl-phenol ether solution (2) and with 1%strength sodium dodecylbenzenesulfonate solution (3), for 20 minutes ineach case. In no case were ,the image parts attacked or underwashed. Ineach case, 25,000 to 30,000 prints of constant quality were obtained ona commercial offset printing press in several tests.

Comparative Example 1

As in Example 1, an aluminum sheet was coated with a 1% strengthsolution of the graft polymer stated in Example 1 in water. Afterdrying, the layer weight was 0.1 g/m². The photopolymerizable materialprepared according to Example 1(a) was, as described under 1(c),laminated with the coated aluminum sheet, exposed, and developed bypeeling off the polyester film. The printing plate obtained remainedstable in water in the ultrasonic treatment but, since the polymer ofthe hydrophilic layer was not crosslinked, was slightly underwashed insolution (2) and rather more strongly underwashed in solution (3), thisbeing evident from underwashing and delamination of individual imagesections. Under the same pressure conditions as in Example 1, only 8,000to 10,000 prints of constant quality were obtained.

EXAMPLE 2

As described in Example 1, a polyester film was coated with thefollowing solution:

2.910 pbw of polyethyl methacrylate as in Example 1,

0.728 pbw of polyethyl methacrylate (Tg: 63° C.; acid number 0) ,

0.140 pbw of polymethyl methacrylate as in Example 1,

2.457 pbw of pentaerythrityl tetraacrylate,

0.203 pbw of bis-cyclopentadienyl-bis-2,6-difluoro-3-(pyrr-1-yl)-phenyl!-titanium,

0.035 pbw of 2,6-di-tert-butyl-4-methylphenol,

0.0175 pbw of Renol Blue B2G,

0.028 pbw of 2-(p-styrylphenyl)-4,6-bis-trichloromethyl-s-triazine and

0.322 pbw of alcohol-soluble Eosin (C.I. 45 386) in

5.12 pbw of butanone,

34.9 pbw of propylene glycol monomethyl ether and

6.98 pbw of γ-butyrolactone

The layer weight was 2.14 g/m² after drying. The further processing wascarried out as in Example 1, but exposure was effected for 9 secondsusing a 500 W incandescent lamp (distance=110 cm). After the polyesterfilm had been peeled off, a negative image remained, which was stable inthe ultrasonic bath under the conditions described in Example 1 and, ona commercial offset printing press, gave over 20,000 prints of constantquality.

EXAMPLE 3

As described in Example 1, a polyester film was coated with thefollowing solution:

2.24 pbw of an alkyl methacrylate copolymer 80° C., acid number 5),

17.9 pbw of polyvinyl butyral (80% of vinylbutyral units, 18% of vinylalcohol units, 2% of vinyl acetate units, Tg 72°-78° C.),

28.0 pbw of pentaerythrityl tetraacrylate,

1.12 pbw of 2-(p-styrylphenyl)-4,6-bis-trichloromethyl-s-triazine,

6.44 pbw of Renol Blue B2G and

0.28 pbw of 2,6-di-tert-butyl-4-methylphenol in

409 pbw of tetrahydrofuran,

279 pbw of propylene glycol monomethyl ether and

55.8 pbw of γ-butyrolactone

The layer weight was 2.7 g/m² after drying.

The aluminum sheet described under 1(b) was coated with a solution of

0.5 pbw of trimethylolpropane,

1.98 pbw of polyacrylic acid (MW=5,800, determined by GPC),

0.03 pbw of p-toluenesulfonic acid and

0.1 pbw of nonylphenol polyethylene glycol-(300) ether in

97.4 pbw of water.

After drying, a layer weight of 0.2 g/m² was obtained. The furtherprocessing was carried out as in Example 1, but exposure was effectedfor 14 seconds. Thereafter, the polyester film was peeled off togetherwith the exposed parts of the layer so that a positive copy having goodresolution remained behind on the aluminum support. The material exposedunder a polyethylene film under reduced pressure without an original for50 seconds was tested in an ultrasonic bath for resistance tounderwashing, as in Example 1. The image sections remained stable in allthree solutions and showed no signs of attack.

Comparative Example 2

A printing plate was produced as described in Example 3, but thealuminum sheet serving as a support was coated with a solution of 2 pbwof the polyacrylic acid stated in Example 3 in 98 pbw of a 0.1% strengthaqueous solution of nonylphenol polyethylene glycol-(300) ether. Thelayer weight was 0.2 g/m² after drying. In the resistance test,substantial damage to the image was evident after the treatment withsolution 3, since the hydrophilic layer did not contain a crosslinkedpolymer.

EXAMPLE 4

As described in Example 1, a polyester film was coated with thefollowing solution:

7.00 pbw of an alkyl methacrylate copolymer as in Example 3,

56.0 pbw of polyvinyl butyral as in Example 3,

36.0 pbw of pentaerythrityl tetraacrylate,

54.0 pbw of dipentaerythrityl pentaacrylate,

3.60 pbw of 2-(p-styrylphenyl)-4,6-bis-trichloromethyl-s-triazine,

10.8 pbw of terephthalaldehyde,

6.50 pbw of Renol Blue B2G and

0.90 pbw of 2,6-di-tert-butyl-4-methylphenol in

1001 pbw of tetrahydrofuran,

683 pbw of propylene glycol monomethyl ether and

137 pbw of γ-butyrolactone

The layer weight was 2.3 g/m².

The aluminum sheet described in Example 1b was coated with a solution of24.0 g of a 40% strength aqueous solution of an acrylic resin (RohmAcrytex W 240) in 576 g of a 0.1 percent strength surfactant solution(disodium salt of decyldisulfodiphenyl ether). The layer weight afterdrying was 0.2 g/m². The positive printing plate produced analogously toExample 3 was treated with a gumming solution for baking (about 5%strength solution of an anionic surfactant in water) and then baked for15 minutes at 200° C., with the result that the lower layer andimage-bearing layer were crosslinked. The printing plate was completelystable after the ultrasonic treatment described in Example 1 and showedno quality changes at all on a commercial offset printing press evenafter 60,000 prints.

EXAMPLES 5 to 12

The photopolymerizable solution stated in Example 1 was applied to afilm and dried as described there. The layer weight was 2.5 g/m². As inExample 1, an aluminum sheet was coated with a solution of 0.8 pbw ofthe graft polymer stated there and 0.2 pbw of one of the crosslinkingcomponents stated in Table I below, in 99 pbw of water, and was driedfor 1 minute at 100° C. The resulting layer weight is likewise shown inTable I.

                  TABLE I                                                         ______________________________________                                                                      Layer Weight                                    Example                                                                              Cross1inking Component (g/m.sup.2)                                     ______________________________________                                        5      H.sub.3 BO.sub.3       0.10                                            6      AlC1.sub.3.6H.sub.2 O  n.d.                                            7      H.sub.4 SiO.sub.4 *    0.06                                            8      H.sub.3 PO.sub.4       0.08                                            9      (NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O                                                        0.12                                            10     H.sub.3 P(Mo.sub.3 O.sub.10.H.sub.2 O                                                                0.10                                            11     Citric acid/p-toluenesulfonic acid**                                                                 0.11                                            12     Polyacrylic acid/p-toluene-sulfonic acid**                                                           0.08                                            ______________________________________                                         *Prepared in situ by hydrolysis of tetraethyl orthosilicate                   **Amount of ptoluenesulfonic acid in each case 0.01 g                         n.d. = not determinable                                                  

Printing plates were produced and processed as described in Example 1.Negative copies of good quality were obtained and were subjected to theresistance test. Table II shows the results. Solutions 1, 2, and 3 aredefined in Example 1.

                  TABLE II                                                        ______________________________________                                        ExampIe  Solution 1    Solution 2                                                                             Solution 3                                    ______________________________________                                        5        ++            ++       ++                                            6        ++            ++       ++                                            7        ++            ++       ++                                            8        +             +        +                                             9        +             +        +                                             10       +             +        0                                             11       ++            ++       +                                             12       ++            +        +                                             V2       ++            -        -                                             ______________________________________                                         -- = Image sections severely damaged                                          - = Image Sections substantially damaged                                      0 = Image sections slightly damaged                                           + = Image sections very slightly damaged                                      ++ = Image sections not damaged                                          

It is intended that the specification be considered as exemplary only.Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein.

What is claimed is:
 1. A process for the production of a lithographicprinting plate, comprisingexposing a material which comprises in thefollowing order:(a) a support suitable for lithographic printing plates,(b) a hydrophilic layer which comprises a crosslinkable organic polymeror is a layer formed by crosslinking an organic polymer, and which has alayer weight in the range from 0.001 to 1 g/m² (c) a photopolymerizablelayer, and (d) a transparent cover film, wherein the adhesion of thephotopolymerizable layer (c) to the hydrophilic layer (b) and/or to thecover film (d) is changed by exposure, imagewise, through the cover film(d), and developing the exposed material by peeling off the cover film(d) from the support (a), wherein the polymer of the hydrophilic layer(b) is crosslinked before or after exposure.
 2. A process as claimed inclaim 1, wherein the polymer is crosslinked by heating.
 3. A process asclaimed in claim 1, wherein the polymer is crosslinked after developmentby heating to a temperature in the range of 180°-240° C.
 4. A process asclaimed in claim 1, wherein the polymer of layer (b) is crosslinked byheating to a temperature in the range of 60°-120° C. prior to layer (c)being coated on layer (b).
 5. A process as claimed in claim 1, whereinthe hydrophilic layer (b) comprises a crosslinkable organic polymerwhich contains hydroxyl, carboxyl, or amino groups as crosslinkablegroups.
 6. A process as claimed in claim 1, wherein the hydrophiliclayer (b) comprises a crosslinkable organic polymer and a crosslinkingagent for the crosslinkable organic polymer.
 7. A process as claimed inclaim 1, wherein the hydrophilic layer (b) comprises a crosslinkableorganic polymer and a crosslinking catalyst for the crosslinkableorganic polymer.
 8. A process as claimed in claim 1, wherein thephotopolymerizable layer (c) comprises,(1) a polymeric binder, (2) acompound capable of free radical polymerization, and (3) a compound or acombination of compounds which is capable of initiating thepolymerization of the compound (2) under the action of actinicradiation.
 9. A process as claimed in claim 1, wherein thephotopolymerizable layer (c) has a layer weight in the range from 0.1 to10 g/m².
 10. A process as claimed in claim 1, wherein the support (a) isa metallic support.
 11. A process as claimed in claim 1, wherein thesupport (a) is a grained and anodized aluminum support.
 12. A process asclaimed in claim 1, wherein the hydrophilic layer (b) comprises acrosslinked organic polymer.
 13. A process as claimed in claim 1,wherein the hydrophilic layer (b) consists essentially of acrosslinkable organic polymer or is a layer formed by crosslinking anorganic polymer, and optionally one or more of a crosslinking agent,crosslinking catalyst, wetting agent, leveling agent, filler, dye, orUV-absorber.
 14. A process as claimed in claim 1, wherein thehydrophilic layer (b) has a layer weight in the range from 0.01 to 0.5g/m².
 15. A process as claimed in claim 1, wherein the material ispositive-working, whereby the exposed portions of the photopolymerizablelayer are removed with the peeled-off cover film.
 16. A process asclaimed in claim 1, wherein the material is negative-working, wherebythe unexposed portions of the photopolymerizable layer are removed withthe peeled-off cover film.
 17. A process as claimed in claim 1, whereinno liquid developers are used.
 18. A process as claimed in claim 1,wherein the image remaining on the support is thermally postcured orbaked.
 19. A process as claimed in claim 1, wherein the polymer of thehydrophilic layer (b) is crosslinked during or after exposure.
 20. Aprocess as claimed in claim 1, wherein the polymer of the hydrophiliclayer is water-soluble and during the process is crosslinked renderingit water insoluble.